1. Field of the Invention
The present invention relates to a continuous casting method which is capable of producing a homogeneous continuous-cast section of a strand, that is directly obtained from molten metal by continuous casting and which has a liquid core, while preventing segregation of impurity elements (e.g. sulfur, phosphorus and manganese in the case of a continuous-cast steel section) from occurring in the center of the thickness of the section.
2. Description of the Prior Art
As marine constructions, reservoirs, steel pipes for transporting oil and gas, and high-tensile wire rods are required to be built of steel materials that have better performance, it has become increasingly important to provide homogeneous steel materials. Theoretically, steel materials should have a uniform composition across their thickness, but steels generally contain impurity elements such as sulfur, phosphorous and manganese, which segregate during casting to provide a brittle steel where they are locally enriched. The use of the continuous casting processes has increased today with a view to achieving higher production rate, yield and saving energy, but pronounced compositional segregation is often observed in the center of the thickness of the strand produced by the continuous casting process. It is highly desirable to reduce the occurrence of center segregation because not only does it significantly impair the homogeneity of the final product but it also causes a serious defect such as cracking by exerting stress on the steel during service of the product or while it is drawn into a wire rod. The mechanism behind the occurrence of center segregation is as follows: the steel that remains unsolidified at the final stage of solidification flows owing to such factors as the force of shrinkage due to solidification and is progressively enriched by washing out the enriched melt present between dendrites in the vicinity of the solid-liquid interface. Therefore, in order to prevent center segregation, it is important to eliminate the causes of fluidization of the residual molten steel. The residual molten steel will flow not only by shrinkage due to solidification but also by the bulging of the strand between rolls and misalignment of the rolls. Of these factors, shrinkage due to solidification is most influential and, in order to prevent center segregation, the thickness of the strand (from which a slab, bloom or billet is obtained) must be reduced by the amount that is necessary to compensate for this phenomenon.
Attempts have been commonly made to avoid segregation by reducing the thickness of a cast steel strand. See, for example, U.S. Pat. No. 3,974,559 wherein the strand being continuously cast is reduced in thickness at a rate not smaller than what is sufficient to compensate for the shrinkage during solidification for the interval during which the temperature of the center of the strand drops from the liquidus line to the solidus line.
However, this method is not completely satisfactory for the purpose of preventing center segregation because little improvement is achieved under certain conditions, or segregation is increased, rather than decreased, in some cases.